A subsea well is typically drilled by drilling a first portion of the well and installing conductor pipe and an outer wellhead housing. Then the well is drilled to a second depth and a first string of casing is installed, the casing being suspended by a high pressure wellhead housing that lands in the low pressure wellhead housing. In one technique, the operator lowers a blowout preventer on a riser and attaches the blowout preventer to the high pressure wellhead housing, then drills the well to total depth.
If the operator is using one type of production tree, referred to herein as a “vertical” tree, he will then complete the well by perforating and installing tubing, with the tubing hanger landing in the high pressure wellhead housing. He then will install a production tree on top of the high pressure wellhead housing.
Alternatively, the operator could land a tubing head or spool on the high pressure wellhead housing before the well is drilled to total depth and connect the blowout preventer to the tubing spool. The operator would complete the drilling through the tubing spool and complete the well by installing the tubing hanger in the tubing spool. The vertical tree would then be landed on the tubing spool.
If the operator is going to use another type of tree, called a “horizontal” or “spool” tree, typically, he would install the tree on the high pressure wellhead housing, then complete the well and land the tubing hanger within the tree.
Another technique involves landing a horizontal tree on the high pressure wellhead housing before employing the drilling riser and blowout preventer to drill the well to total depth. In this technique, the first and second portions of the well are drilled and at least the first string of casing installed without the use of a blowout preventer. The horizontal tree lands on top of the high pressure wellhead housing, and the drilling riser is connected to the upper end of the horizontal tree. The drilling riser may have a subsea blowout preventer, or it may be of a high pressure type with a surface blowout preventer located on the drilling vessel. The operator drills through the tree to total depth and runs the casing through the blowout preventer and drilling riser. The operator installs the tubing hanger in the horizontal tree.
Both a tubing spool and a horizontal tree have one or more ports that lead from the bore to the exterior. These ports may include tubing annulus ports that communicate with the annulus surrounding the string of tubing. Also, ports exist for supplying hydraulic fluid pressure to mating ports in the tubing hanger for a downhole safety valve. There may be ports for electrical lines for downhole sensors, as well. A horizontal tree also has a production outlet port leading from the bore, but a tubing spool would not have a production outlet port.
A wear bushing will be fitted within the bore of a horizontal tree or tubing spool while at the surface to protect the sealing surfaces within the bore during subsequent drilling. The wear bushing is a tubular sleeve that will cover all of the ports leading into the bore and all sealing surfaces in the bore. Normally the wear bushing is sealed to the tree bore above and below the ports to prevent entry of drilling mud and debris into the ports.
Industry practice requires that the valves leading to these various ports be tested after the tree or tubing spool has been installed on the high pressure wellhead housing. Normally, each port will have a small diameter test passage that leads to it for supplying hydraulic fluid pressure to the port between the valve and the wear bushing. The test pressure exerts an inward force on the wear bushing.
With a drill-through tubular member, such as a horizontal tree or tubing spool, the wear bushing is quite thin so as to maintain a full bore diameter for the passage of casing, casing hangers, drill bits and the like. The test pressure could cause buckling and collapsing of the wear bushing. U.S. Pat. No. 6,966,381 discloses placing a test sleeve within the wear bushing to prevent collapse of the wear bushing during pressure testing of the port valves, then retrieving the test sleeve before commencing drilling.
Another test procedure required is to test the blowout preventer after it has landed and before drilling occurs. A blowout preventer has a number of closure members that will close around conduit and also close the full bore. Also, choke and kill lines extend alongside the riser from the surface to a point below one or more of the closure members of the blowout preventer. One manner of testing the blowout preventer is to lower a test tool on a string of drill pipe through the blowout preventer. The test tool has a packer element that seals to the bore of the tubular member on which the blowout preventer is secured. The operator closes one of the closure members around the drill pipe and supplies fluid pressure through one of the choke and kill lines to the sealed chamber defined by the closure member and the packer element. If the test is successful, the test tool is retrieved and the drill string ran back in with a drill bit.